the muscles make it hard to hold the guitar correctly.
How are you "correctly" holding the guitar? Like Segovia? Or are you referring to the dozens of different ways of having your guitar up around your neck like Roy Buchanon, below your waist like early SRV, or anywhere in between?
Holding the guitar, or at least the height of the strap, is very individual. But classical guitarists usually hold it specifically, like Segovia. In either case, muscles do not get in the way.